Developments have been recently active in semiconductor devices using wide bandgap semiconductors. A gallium-nitride (GaN)-based material is particularly a promising wide bandgap semiconductor. GaN, which has higher resistance to dielectric breakdown and higher electron mobility than silicon (Si), is receiving attention as the material of a substrate for fabricating a rectification element and a switching element, which are power devices. GaN can be used to manufacture high-voltage and low-resistant field effect transistors (FETs), Schottky barrier diodes (SBDs), and PN diodes (PNDs).
GaN, when used as a substrate material, unfortunately disfavors formation of a highly active p-type diffusion layer through ion implantation, unlike Si and silicon carbide (SiC), which are typical materials for a power device. Hence, unlike Si and SiC, GaN cannot form a p-type field-reducing structure, typically, a p-type guard ring (also referred to as a p-type field-limiting ring or p-type FLR for short) structure through impurity implantation. A possible way to form the p-type field-reducing structure is forming and patterning a p-type epitaxial-growth layer instead of implanting acceptor impurity ions. Moreover, the field-reducing structure is possibly formed using a field plate structure.
For instance, Patent Document 1 discloses a horizontal SBD. This diode has a field plate structure provided with a stair structure to further distribute field concentration. With such a structure, Patent Document 1 attempts to enhance breakdown voltage.
Further, Non-Patent Document 1 discloses a vertical PN diode. This diode has a slanted filed plate structure consisting of a combination of a mesa-shaped GaN layer, and a spin-on-glass (SOG) film, which exerts a planarization effect. This structure enables a structure for reducing field concentration to be formed through a simple process.
Still further, Non-Patent Document 2 discloses a vertical junction-barrier-Schottky (JBS) diode. This diode has an n-GaN layer and a p-GaN layer on the n-GaN layer. The p-GaN layer is partly removed, bringing an anode electrode into contact with the n-GaN layer and the p-GaN layer. In this structure, the p-GaN layer reduces the electric field at the Schottky interface between the anode electrode and the n-GaN layer. With such a structure, Non-Patent Document 2 attempts to enhance breakdown voltage. The anode electrode has an end spaced away from a semiconductor region by a passivation layer.